A conventional current controller includes a current detecting device for detecting current values of respective phases of a three-phase inverter on the basis of voltage decreases of shunt resistors connected in series to lower arm elements of the respective phases and controls an electric current for driving a motor. Among such current detecting devices, provided has been a current detecting device comprising a current value determining part, which employs a reverse signal value of the sum of current values of two phases as a current of the other phase of one lower arm element having a short on-period among the lower arm elements, wherein all phase currents are detected with high accuracy during a current phase period in which one lower arm element incapable of detecting a shunt resistor due to its arrangement on the lower arm side is only off and a current phase period in which a short on-period of the lower arm element causes difficulty in detection with high accuracy although the lower arm element is on. (Refer to Patent Reference 1, for example.)
Further, as a current offset correction method of a current controller, provided has been the following one. In the current offset correction method, when a state of 50% duty ratios of all PWM signals PWMU, PWMV and PWMW denoting that phase currents iu, iv and iw are 0 continues for a predetermined time sufficient to lose electromagnetic energy accumulated in a motor, the phase currents iu, iv and iw are read and memorized as current offset quantities, which are used thereafter to conduct offset compensation for the detected phase currents iu, iv and iw, so as to achieve a simple and accurate detection of the phase currents iu, iv and iw. (Refer to Patent Reference 2, for example.)    Patent Reference 1:
JP-A-2003-164159 (Pages 3 and 4 and FIG. 5)    Patent Reference 2:
JP-A-2003-164192 (Pages 4 and 5 and FIGS. 5 and 6)
Related art will be described hereinafter, using the drawings. FIG. 4 illustrates a general structure of a current controller having a current detecting device such as one disclosed in Patent Reference 1. In FIG. 4, 1 denotes a current controller, 2 denotes a three-phase PWM inverter part, 3 denotes a switching element driving circuit, 4 denotes an A/D converter part, 5 denotes a current control operating part, 6 denotes a modulated wave command creating part, 7 denotes a current detection value operating part, 8 denotes a carrier wave generating part and 9 denotes a PWM signal generating part. Further, 101 denotes a commercial power source, 102 denotes a converter part, 103 denotes a smoothing capacitor, 104 denotes a motor, 105 denotes a position detector and 106 denotes an upper controlling part. Adding the current controller 1 to the above forms a general structure of a motor controlling device. Moreover, 21 denotes a switching element and 22 denotes a freewheel diode. 23 denotes a shunt resistor inserted between a switch S2 on a negative side (Nch) and a negative side (Nch) of a DC bus for detecting an electric current. 24 denotes a charge pump circuit for using a negative side pulse of the switching element 21 to make a driving power supply for the switching element 21.
The freewheel diode 22 connected in parallel reversely to an IGBT transistor 21 is used for forming a switch S1. Two of S1 and S2 connected in series form a pair for one phase. Three pairs of the above are used for three phases. The charge pump circuit 21 is connected from a DC power source +Vs to the switching element driving circuit 3 through diodes and a capacitor, which are connected respectively in a forward direction with respect to the respective phases. The switching element 21, the freewheel diode 22, the shunt resistor 23 and the charge pump circuit 24 form the three-phase PWM inverter part 2.
First of all, described will be an operation of a general motor controlling device. The upper controlling part 106 performs position control, speed control and torque control in accordance with a position feedback signal, which is a position detection signal in the position detector 105, and a command for operation from the outside to output a current command to the current control operating part 5. The converter part 102 converts AC power of the commercial power source 101 into DC power. The three-phase PWM inverter part 2 converts DC power into AC power in accordance with a PWM signal from the PWM signal generating part 9 to supply the motor 104 with power. In such a series of operation, the motor controlling device controls the motor in accordance with a command for operation from the outside.
Now, described will be operations of the respective parts of the current controller 1. The A/D converter part 4 detects voltages at the both ends of the shunt resistor 23 in the three-phase PWM inverter part 2 to detect an electric current flowing in the shunt resistor 23. The A/D converter part 4 A/D converts the detected voltages at the both ends as current feedback signals to output the signals to the current detection value operating part 7. The current detection value operating part 7 carries out a current detection value operation of the current feedback signals from the A/D converter part 4 to output the current detection value to the current control operating part 5. Further, the current detection value operating part also performs both of offset value detection and an offset correcting operation, which are based on a current detection value, in the case of carrying out an offset correction of the current detection value.
The current control operating part 5 performs a current control operation on the basis of a current command from the upper controlling part 106, a current detection value from the current detection value operating part 7 and a position feed back signal to output a voltage command to the modulated wave command creating part 6. The carrier wave generating part 8 outputs a carrier wave in accordance with carrier wave frequency and carrier wave amplitude. The modulated wave command creating part 6 limits a modulation ratio in the case that the voltage command is larger than the carrier wave amplitude. The modulated wave command creating part 6 then creates a modulated wave command so as to output voltage according to the voltage command to output the modulated wave command to the PWM signal generating part 9. The PWM signal generating part 9 compares the carrier wave and the modulated wave command to generate a pulse signal, and further, generate an inversion signal of the pulse signal. After the above, the PWM signal generating part generates PWM signals for two of positive and negative sides of the switching element.
The PWM signals have dead time so that the switching elements on the positive and negative sides of the same phase would not turn on simultaneously since a DC power source is short-circuited in the case of the switching elements simultaneously turning on. Moreover, generally in accordance with the voltage command or the modulation ratio command, compensation of voltage varying according to the dead time is carried out on the basis of a current command, a current detection value, a current estimation value and the like. This allows the voltage command to be accorded with the actual voltage. The switching element driving circuit 3 controls ON/OFF of the switching element 21 in accordance with the PWM signal to supply the motor 104 with power.
Now, described will be an offset correcting operation in the related art. FIG. 5 is a flowchart showing a current offset quantity detecting operation in the related art. First, duty ratios of PWM signals of the respective phases are read (S100). The duty ratios of PWM signals, which have been read, are judged whether or not all of the duty ratios are 50% (S102). In the case that all the duty ratios are judged to be 50%, it is judged whether a state that all the duty ratios are 50% continues for a predetermined time or not (S104). When it is judged that the state continues for the predetermined time, operations of current offset quantities of the respective phases are executed (S106, S108 and S110). In the above context, the duty ratios of the PWM signals of the respective phases are assumed to be values ignoring the dead time.
FIG. 6 is a flowchart showing an operation of carrying out a current offset quantity operation in the related art. First, the phase currents iu, iv and iw of the three phases are read during the period that a lower arm element at the subsequent time is on, and then, the read phase currents iu, iv and iw are set as the present time values of the offset quantities of the respective phases (S200). Second, the sum of the present time values of the respective offset quantities, which are respectively obtained during the period that a lower arm element at the N-th (wherein N denotes an integral value) time preceding to the present time is on, is calculated (S202 and S204). The sum is divided by N to calculate an average offset value for every phase (S206) and memorize the calculated value (S208).
FIG. 7 is a flowchart showing a current offset correction operation using the current offset quantity in the related art. First, the phase currents iu, iv and iw of the three phases are read with predetermined timing (S300). Memorized values of the current offset quantities are then subtracted from the read phase currents iu, iv and iw of the three phases, individually, to obtain offset compensation phase currents iu′, iv′ and iw′ (S302). The obtained offset compensation currents iu′, iv′ and iw′ are outputted as the new current detection values (S304).
As described above, in the conventional device and the current offset correction method of the same, carried out current offset quantity detection, current offset quantity operation and current offset quantity correction in the current detection value operating part 7 to perform current control on the basis of the new current detection value obtained by correcting variation of the offset quantity of the current detection system.